A sawmill carriage is generally made up of movable knees providing a vertical surface against which a log rests may rest against and movable dogs used to clamp and hold the log while the carriage reciprocates back and forth along a track parallel to a saw blade. Before each pass, the log may be moved, or “jogged” toward or away from the saw blades or set. The jogging is generally accomplished by horizontal movement of the knees toward or away from the saw blades, to the desired thickness of the cut board desired to be produced from the log. The board is cut as the log advances through the saw. This process continues until the operator or “sawyer” decides to unload what is left of the log, the left-over piece generally referred to as a “cant” or “dogboard.”
One dog, a “tong dog” system, comprises of an upper dog, a lower dog, a mechanism to bring the dogs together and a mechanism to move the dogs horizontally away from or towards the vertical surface of the knee. The knees themselves advance toward or away from the saw line, and is most commonly driven by a chain or rack gear.
The chain drive mechanism has several advantages over the rack gear system, making it the desirable mechanism by which to move the knees. The tong dog systems that use a rack gear to position the knees relative to the sawline are generally difficult to replace, not easy to adjust, and must have slack built-in even when the system is new. Comparatively, a chain drive system is easily adjustable, durable and easy to replace as wear necessitates.
A chain drive system must have a mechanism by which to tension the chain. As the chain drive system components and chain rollers wear, the chain will loosen and must be adjusted to prevent excessive slack. The chain tensioning mechanism adds to the complexity of the sawmill carriage, increasing the number of components necessary and necessitates a carriage structure sufficient for the force transfer from the log though the dogs and dog adjustment mechanism, through the knee components to the chain drive. A need exists for a chain drive system having a fewer components, is simple to service and adjust and provides for an efficient force transfer from the log to the chain drive to reduce number of stressed components in the sawmill carriage.